NOR or NAND flash memories have been typically used as semiconductor nonvolatile memories for data storage. These semiconductor nonvolatile memories have achieved an increase in capacity by miniaturizing memory devices and drive transistors; however, since a high voltage is necessary for writing and erasing, and the number of electrons that are to be injected into a floating gate is limited, limitations in miniaturization has been pointed out.
At present, resistance change memories such as ReRAMs (Resistance Random Access Memories) and PRAMs (Phase Change Random Access Memories) have been proposed as next-generation nonvolatile memories that are capable of transcending the limitations in miniaturization (for example, refer to PTL 1 and NPTL 1). These memories have a simple configuration in which a resistance change layer is provided between two electrodes, and it is considered that atoms or ions are moved by heat or an electric field to form a conductive path, and thus a resistance value of the resistance change layer is changed, thereby performing writing and erasing. More specifically, there has been proposed a storage device using a transition metal element, a chalcogen element, and copper (Cu) that tends to cause ion conduction (for example, refer to PTL 2).
In addition to miniaturization of the memories as described above, multi-valuing of memories is another method of achieving an increase in capacity of memories that perform writing and erasing by resistance change. When multi-valuing of the memories, i.e. multi-value recording of 2 bits (4 values), 3 bits (8 values), or the like per device is enabled, the capacity is increased twofold or threefold.